This invention relates generally to compressors, and more particularly to hermetic compressors of the fractional horsepower type used in household appliances such as refrigerators and freezers.
The need for increased energy efficiency for household appliances is particularly great for these types of appliances because they use such a large amount of the total electrical energy consumption in the typical household. One of the areas where much improvement has been obtained in these units is the hermetic compressor, which has seen considerable energy efficiency improvement in recent years. While much of the improvement has been in the electric motor portion of the compressor, there still remains further room in the area of volumetric and compression efficiency of the reciprocating piston compressor.
One of the factors affecting the volumetric efficiency of these compressors is the clearance or re-expansion volume of the pumping cylinder, which is defined as the volume of space within the pumping cylinder when the piston is at top center or the end of its pumping stroke. This space consists essentially of the space between the piston face and the valve plate on which the suction and discharge reed valves are mounted as well as the volume of the discharge port in the valve plate, since the discharge valve reed valve is on the outer side of the valve plate, while the suction valve is on the inner side of the valve plate so that the volume of the suction port is outside of the clearance volume. The ideal compressor would have no clearance volume, and generally, the greater the clearance volume, the lower the efficiency of the compressor. The reason that clearance volume adversely affects efficiency is that this volume constitutes gases that require additional work or energy for compression on the working stroke of the piston, and this energy is only partially recovered on the suction stroke as the cylinder is refilled through the suction port. Thus, reduction of the clearance volume will increase the efficiency of the compressor as long as other factors are not also adversely affected.
Since the clearance volume consists mostly of the above-described two components, efforts to reduce this volume have taken the form of minimizing the distance between the piston face and the valve plate, or more specifically, the valve sheet incorporating the suction valve reed. As for the volume of the discharge port, the diameter cannot be reduced below a certain minimum because this would increase the restriction to discharge flow, and the length of the port must be sufficient in terms of valve plate thickness for the necessary strength to resist the forces of the compressed refrigerant. While some port length reduction has been accomplished by recessing the discharge valve in the valve plate as disclosed in U.S. Pat. No. 4,723,896, granted Feb. 9, 1988 to J. F. Fritchman and assigned to the assignee of the present invention, strength requirements still need enough valve plate material that the discharge port remains a substantial portion of the total clearance volume.
Because of the problem of tolerances in the various parts, the clearance volume from the spacing between the piston end face and the valve sheet has been carefully controlled by a selective thickness fit for the gasket located between the end surface or face on the cylinder block and the valve sheet. It has been found that if this spacing is reduced too much, the compression efficiency is actually reduced. This has been found to be the result of the fact that the discharge port is not only a fraction of the size of the cylinder bore, but also is usually located off the center of the cylinder axis. Thus, as the piston reaches the end of the compression stroke and the clearance space approaches the minimum, the compressed refrigerant gas must flow laterally across the piston face to reach the discharge port. If the spacing between the piston face and the valve sheet is reduced too much, the compressor efficiency is actually reduced because some of the compressed gas becomes effectively trapped in the clearance space since it does not have time with the high speed of the compressor to flow toward and reach the discharge port before the piston reverses direction. As a result, reducing the clearance space at the piston face below a certain minimum may actually reduce the compression efficiency of the compressor by increasing the mass of the gas compressed and re-expanded within the clearance volume.